Coating surfaces of iron or steel



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.lytic or partially Patented May 3, 1932 UNITED STATES PATENT, OFFICE WILLIAM 1a. PHILLIPS, or BIRMINGHAM, Aim GUY M. coLn'or nnraoir, MICHIGAN,

ASSIGNORS TO GENERAL PORATION OF DELAWARE MOTORS CORPORATION, OF DETROIT, MICHIGAN, A COR- No Drawing.

The invention relates'to the formation of protective coatings upon articles having surfaces of iron-or steel and, more particularly, to the deposition'of such coatings by electroelectrolytic methods. To this end the invention contemplates the exposure of the surfaces in an electrolytic bath in which the article is made the cathode, the bath being of such composition that an in soluble phosphatic coating is formed directly upon the surfaces.

The bath used is one containing a' phosphate or phosphates, preferably of zinc, together with a small amount of free phosphoric acid. It may be prepared in various Ways and with considerable variation in com-.

position, the following being given as typical examples.

in preparing the bath a zinc oxide may be dissolved in phosphoric acid to form a phosphate or'phosphates of zinc, an excess of acid being preferably used in forming the solution, which excess is afterward neutralized to a certain point by addition of an alkali, for example, sodium hydroxide or carbonate. lln following this prbcedure, a considerable range of concentration is permissible, deposits of satisfactory character having been obtained with solutions made with from one to fifteen grams of zinc oxide per liter of bath. The following is suggested as desirable: For one liter of bath, dissolve 5 grams of Zn() in 23 c. c. of syrupy (about 85%) 15 1 0 neutralizing the excess of acid, after solution is completed, by addition of NaOH until a White precipitate just begins to form. This leaves in the bath a very slight excess of phosphoric acid.

ith this bath maintained at near the boiling temperature, with a steel anode and the steel article to be coated connected as cathode, supplying current at a current density at the cathode of 10-15 ainperes per square foot, a dense black coating of phosphate. or phosphates is obtained on the cathode in a very few minutes.

Other compounds may be employed in pro paring the bath, in forming a phosphate or phosphates of zinc as in the above illustration, provided the compound used does not Application filed January 26, 1929. Serial No. 335,384.

introduce an anion which would interfere with the formation of the phosphatic coating. Zinc carbonate, for example, might'be I used in lace of zinc oxide.

The ath may also be prepared by dissolving in water other phosphates of zinc, for example, the acid zinc phosphate, Zn (H PO,)' which, as is well known, may be obtained by dissolving metallic zinc in concentrated orthophosphoricacid.- The con- 00 centration suggested is from 5 to 8% of phosphate. The required quantity of free acid can'be added to maintain the acidic character of the bath.

The content of free phosphoric acid should as be controlled Within relatively close limits. The amount which we have found desirable is in the neighborhood of 0;05 to 0.13%. An excessive content of free acid is likely to result in the deposition of more metallic zinc in the coating, whereas, with the proper acid content, the black phosphatic coating alone is obtained.

The current density also affects the character of the deposit. An increase of current density, tends to cause deposition, of free metallic zinc along with the black phosphatic coating, so that control of the current density is necessary if the latter coating only is desired. The presence of metallic zinc is not at all detrimental if the coated surface is not tobe further treated, or if the further treatment is to consist merely in applying a coating of oil or the like. If, however,/the" article is to receive a finish of such coating 86 material as paint, lacquer or enamel, it is desirable that no free metallic zinc should be present on the surface to be so coated.

In treating the surface forreception of such finishes aspaint, lacquer or enamel, the 90 treatment should be such as to leave on the exposed surface only the phosphatic coating. Should the control of acidity and current density'not be such as to avoid the deposition of zinc, the free zinc may be converted into 05 the phosphatic coating by mere exposure to the bath without passage .of current. A reaction takes place between the zinc and the constituents of'the bath affected possibly by the presence of iron,resulting in the forma permitting tion of the coating of insoluble phosphate or phosphates on the exposed surfaces. It may be desirable, in some cases, to su ply the current intermittently, thus provi 'ng in tervals during which the reaction may proceed between the electro-deposited zinc and the ingredients of the bath.

While a steel anode is regarded as pr ferfor example, has been used. The v ltage varies with the concentration of the co stituents ofthe bath. From one to six volts have been found satisfactory. The bath should be maintained at a high temperature, near be made from the exact details above set forth, and therefore the inbe understood as being revention should not stricted' to such specific disclosure except as required by the terms of the appended claims. We claim:

1. Theprocess of treating articles having surfaces of iron or steel comprising exposing such surfaces in a bath containing a solution of zinc phosphate, and passing current through the bath and eifectan electrolytic deposition of zinc upon such surfaces.

2. The process of treating articles having surfaces of iron or steel to produce thereon a 40 posing the surfaces to the action of a bath containing a salt from" which zinc may be electrolytically deposited upon said surfaces and phosphoric acid compound adapted to react with such deposit to produce such prd tective coating, and passing current through such bath and article to effect deposit of zinc on said surfaces.

3. The process of treating articles having surfaces of iron or protective coating comprising electrolytically depositing zinc upon sa1d surfaces and able, it is not'considered essential, asinc,

phosphatic protective coating comprising exwhile reaction takes place between the phosphoric acid compounds thereof and the electrode p osited zinc.

5. he process of forming a phosphatic prising exposing the articles to a bath containing a solution of zinc hosphate and free phosphoric acid, the acid being within the approximate limits of 0.05 to 0.13 percent, passing current through the bath and the article as cathode with a voltage sufficient to desit metallic zinc on-the article and COIltlIlll. mg the exposure to the bath to cause reaction of the deposited zinc to form zinc phosphate on the surface.

6. A ferrous metal article coated with a mixture of metallic zinc and zinc phosphate.

In testimony whereof we afiix our signatures.

' WILLIAM M. PHILLIPS.

GUY M. COLE. a

the article as cathode to steel to produce thereon a simultaneously exposing said zinc deposit to the action of phosphoric acid compounds effective to convert such salts containing zinc.

4. The process of forming a zinc-containing protective coating upon articles having iron or steel surfaces comprising exposing such surfaces in a bath containing phosphoric acid and one or more zinc compounds thereof, passing current through the bath and the article as cathode to deposit metallic zinc thereon, interrupt-ing the deposition of zinc and the article to remain in the bath deposit into protective coating upon articles of ferrous metal com- 

